For manufacturing the above-mentioned parts and elements, martensitic stainless steels, work-hardenable austenitic stainless steels, precipitation-hardenable stainless steels, etc. have conventionally been used.
Martensitic stainless steels are hardened by quenching from the austenitic state at an elevated temperature to cause martensitic transformation. Steels of SUS 410, 410J, 420J1, 420J2, 440A, 440B, 440C, etc. are typical examples of these steels, which have conventionally been used. Although these steels are low in strength and toughness in the annealed state, considerably high strength and toughness are attained by quenching and tempering. Therefore, these steels are widely used as inexpensive materials.
However, as martensitic stainless steels are not satisfactory for use in which high corrosion resistance is required, in such a field, work-hardenable austenitic stainless steels are used. These steels are Cr-Ni austenitic steels which are in the metastable state at ordinary temperatures and are hardened by cold rolling. The hardened steels are of two phases consisting of austenitie and martensite and therefore excellent in strength and ductility and also excellent in corrosion resistance. Typical examples of these steels are SUS 301, 304, etc. The strength of these steels depends upon the degree of cold working as stipulated in JIS G4313 and intensive cold working is required in order to attain high strength.
Precipitation-hardenable stainless steels contain precipitation-hardening elements and are hardened by heat-treatment, and therefore afford articles of good shape. Therefore, these steels are employed when shape requirements of products are strict and corrosion resistance is an important factor.
Typical examples of these steels are SUS 630, which contains Cu, and SUS 631, which contains Al. The former is hardened by solution treatment followed by aging during which a Cu-rich phase is precipitated. But the hardness thereof is 140 kgf/mm.sup.2 at the highest. The latter is hardened by first subjecting to solution treatment, then transforming tbe metastable austenite phase partly or wholly to the martensite phase by cold working, for instance, and thereafter precipitating a Ni.sub.3 Al intermetallic compound by aging. This can provide considerably high strength materials.
As a method for transforming the austenite phase of SUS 631 to martensite phase and then aging it, treatments such as TH 1050, RH 950, CH, etc. can be resorted to. But the strength attained by the former two treatments is 130 kgf/mm.sup.2 at the highest, while a strength as high as 190 kgf/mm.sup.2 can be attained by the CH treatment. In the CH treatment, the steel is first subjected to cold working to convert the austenite phase to the austenite-martensite two phases as in the case of work-hardenable stainless steels, and is thereafter subjected to aging. The strength after the cold working is around 150 kgf/mm.sup.2, depending on the degree of cold working. But the above-mentioned high strength is attained by precipitation of the Ni.sub.3 Al intermetallic compound when the steel is age-hardened.
Of the above-described stainless steels, martensitic stainless steels must be subjected to quenching and tempering in order to attain strength and toughness. The heat treatments are troublesome. In quenching, materials are heated to a high temperature (950.degree.-1100.degree. C.), wherefrom they are quenched. Rapid mertensitic transformation deteriorate shape of treated articles. In order to prevent such trouble, a special heat treatment such as press-quenching is required.
In the case of austenitic stainless steels, high degree cold working is required in order to attain high strength. But if high strength is attained, ductility is sacrificed, and the shape of sheet products and strip products is often deteriorated.
Further, in the case of precipitation-hardenable stainless steels, SUS 630 does not attain high strength, and SUS 631 often devlops surface roughness and is impaired in toughness and ductility because the steel contains 0.75-1.50% Al which has a strong affinity for oxygen and nitrogen, and alumina type inclusions are formed during the steel-making and coagulated inclusions of AlN are formed when the steel is cast.
Japanese Laid-open Patent Publication No. 52-007317 disclose a steel substantially contained in % by weight, C: .ltoreq.0.02%, S: .ltoreq.1.00%, Mn: .ltoreq.2.00%, P: .ltoreq.0.040%, S: .ltoreq.0.003%, Ni: 5.00-8.50%, Cr: 16.00-21.00%, Cu: 0.50-4.00%, N: &lt;0.20%, O: .ltoreq.0.015% and the balance being Fe and unavoidable impurities. This steel is for compression forming and, therefore work-hardenability and formation of martensite are restricted by reducing C content, increasing N and adding Cu and reducing Si. That is, hardness of the resulting products are not satisfactory.
Japanese Laid-Open Patent Publication No. 56-077364 discloses a steel comprising, by weight C: .ltoreq.0.15%, N: .ltoreq.0.15%, Si: 0.ltoreq.1.5%, Mn: 0.5-2.0%, Ni: 5.0-9.0%, Cr: 13.0-20.0%, Cu: 1.0-4.0%, and the balance being Fe and unavoidable impurities and having the Md.sub.(30) (.degree.C.) value of -30.degree.-80.degree. C., said Md.sub.(30) (.degree.C.) being defined as ##EQU1##
The Md.sub.(30) (.degree.C.) is the temperature at which 30% cold-worked super-cooled austenite transforms into martensite of 50% and represents austenite stability (instability). This steel is intended for a spring material as well as the present invention. However, this steel is not satisfactory in the balance of strength and elongation. This is because the Mn content is rather high, the Si content is rather low and S is not restricted.
U.S. Pat. No. 4,378,246 by the inventors including two of the inventors of the present invention discloses a martensitic precipitation-hardening type stainless steel for spring comprising in % by weight more than 0.03% but not more than 0.08% of C, 0.3 to 2.5% of Si, not more than 4.0% of Mn, 5.0 to 9.0% of Ni, 12.0 to 17.0% of Cr, 0.1 to 2.5% of Cu, 0.2 to 1.0% of Ti and not more than 1.0% of Al, the balance being Fe and having a specifically defined restricted austenite stability A' of less than 42, said A' being defined as ##EQU2## having a specifically defined Cr equivalent/Ni equivalent ratio of not more than 2.7, said ratio being defined as ##EQU3## and further having a specifically defined hardness increase by aging .DELTA.Hv of between 120 and 210, said .DELTA.Hv being defined as ##EQU4## This steel is genuinely martensitic precipitation-hardenable steel. The fact is represented by the A' value less than 42, the rather high Mn content and addition of precipitate-forming elements such as Ti and Al. The A' value is an index which represents existence of the residual austenite after solution treatment. When this value is less than 42, the steel is simply martensitic.
It is not that ductility is not considered as the upper limit of .DELTA.Hv is somewhat restricted. However, ductility of this steel is not sufficient.